Jesus himself tells us...“Because to you it has been given to know the mysteries of the kingdom of heaven, but to them, it has not been given…Therefore, I speak to them in parables, because seeing they do not see, and hearing they do not hear, nor do they understand. And in them the prophecy of Isaiah is fulfilled, ‘Hearing you will hear and not understand and seeing you will see and not perceive; For the hearts of this people have grown dull. Their ears are hard of hearing and their eyes they have closed, lest they should see with their eyes and hear with their ears, lest they should understand with their hearts and turn, and I should heal them’.” (Matt 13: 10 - 15)

“And He said, ‘To you it has been given to know the mysteries of the kingdom of God, but to the rest I speak in parables, that ‘Seeing they may not see and hearing they may not understand.’” (Luke 8: 9 -10).

"But when He was alone, those around Him with the twelve asked Him about the parable. And He said to them, “To you it has been given to know the mystery of the kingdom of God; but to those who are outside, all things come in parables, so that Seeing they may see and not perceive, And hearing they may hear and not understand; Lest they should turn And their sins be forgiven them.’” (Mark 4: 10 - 12)

i.e The reason some people do “not understand” parables is that it has “not been given to them” to be able to do so. In quoting Isaiah the intent behind the parables according to Luke and Mark is that in “seeing they will not see and hearing they will not understand.” To those who "have been given" understanding, they will recognize that everything they have, including their understanding, is by God's grace alone.